兄弟会和姐妹会（英语：Fraternities and Sororities）一般指的是世界性组织。包括光明会和玫块十字会。通常以希腊字母命名，并一起通称希腊社团（Greek Organization）。

在拉丁语中，Frater和Soror分别代表兄弟（Brother）和姐妹（Sister）的意思。兄弟会是一种以兄弟情谊为基础招收在校学生的学生社团组织。在北美，兄弟会一般意指大学中的希腊社团，属学生社团组织。但在欧洲部分类似性质的社团法人（Corporation）也适用此称呼。少数高中也有类似的组织。

A fraternity (from Latin frater: "brother"; "brotherhood"), fraternal order or fraternal organisation is an order, organisation, society or a club of men associated together for various religious or secular aims. Fraternity in the Western concept developed in the Christian context, notably with the religious orders in the Catholic Church during the Middle Ages. A notion eventually further extended with the middle age guilds, followed by the early modern formation of gentlemen's clubs, freemasons, odd fellows, student fraternities and fraternal service organisations. Members are occasionally referred to as a brother or - usually in religious context - Frater or Friar.